Infrared (IR) lasers, which operate in the wavelength range of 700-4,000 nm, and in the 1,300-3,000 nm in particular, have recently become popular in laser radar, three-dimensional scanning, targeting, and communication applications. Such applications require the laser to operate under Q-switching, which can generate a laser pulse with duration on the order of tens of nanoseconds, and a peak power on the order of a megawatt. Q-switches are described by R. Wu, S. Jiang, M. Myers, J. Myers, S. Hamlin, SPIE, Vol. 2379, Solid State Lasers and Nonlinear Crystals, 1995. A Q-switch has the effect of a shutter moving rapidly in and out of the light beam, altering the normal “Q factor”. The Q factor represents the energy storing efficiency of the resonator, namely the ratio of stored energy to dissipated energy. The Q switch maintains a low Q factor until a high level of energy is stored within the cavity. When the normal Q factor is restored, the laser energy is released in the form of a very short pulse of high intensity.
A Q-switch can be an active or a passive device. Active Q-switches utilize opto-mechanical techniques (e.g., rotating prisms), electro-optical techniques (e.g., beta-BaB2O4 (BBO) crystal, nonlinear optical crystal or Frustrated Total Internal Reflection (FTIR) methods), acousto-optical techniques (e.g., a change in the light Bragg deflection upon the application of sound wave) and magneto-optical techniques (e.g., a change in optical properties of a crystal in the presence of an external magnetic field). Further elaboration can be found in R. Wu, J. Myers, S. Hamlin, Presentation at the OSA Advanced Solid-State Laser Conference, (ASSL) 1998. These Q-switches are controlled or driven by external forces which are not entirely based on optics, and therefore usually have a relatively slow operation time and a limited bandwidth. Active Q-switches require external electronics, additional optics, and magnetic field or acoustic wave accessories, which increase the physical size and can also add expenditures to the cost of the switching device.
The operation of a passive Q-switch is due to the intrinsic properties of the material. As a result, a passive Q-switch produces a fast response time (on the order of picoseconds-nanoseconds) and a relatively high output power. Various applications require reliable, simple and compact laser systems with short and powerful laser pulses. The technique of passive Q-switching is both compact and simple, because it requires only a saturable absorber introduced in the laser cavity, without any auxiliary electronics. A saturable absorber is a material whose absorption coefficient drops at high levels of incident radiation. Since such a switch has no external control, it consequently has a high capability for integration, in terms of manufacturing, operation, fabrication cost, system size, and weight.
Various materials have been shown to operate as passive Q-switches in the eye-safe wavelength region. For instance, the fluorides (Er3+:CaF2, Er3+:Ca5(PO4)3, U2+:CaF2, U2+:BaF2, U2+:SrF2) based on Co2+ ions doped semiconductors, single crystals, and glass ceramics, have been tested as a saturable absorber operating at 1.54 μm, and demonstrated sufficient performance as a passive Q-switch.
Semiconductor crystals can function as a saturable absorber over wide spectral regimes, when the size of the crystals is in the nanometer range. PbS and PbSe nanoscaled crystals embedded in phosphate and silicate glasses were experimentally employed for passive Q-switching of Erbium doped glass (Er:glass) laser systems. Such materials are described in J. F. Philipps, T. Topfer, H. Ebendorff-Heidepriem, D. Ehrt, R. Sauerbrey, N. F. Borrelli, Appl. Phys. B 72, 175-178 (2001); A. M. Malyarevich, V. G. Savitsky, I. A. Denisov, P. V. Prokoshin, K. V. Yumashev, E. Raaben, A. A. Zhilin, and A. A. Lipovskii, Phys. Stat. Sol. (B) 224, No. 1, 253-256 (2001).
The aforementioned materials were developed to work only with a Er:glass laser (operating at 1.54 μm), which was until recently the only accessible laser in an eye-safe wavelength spectral regime. However, lately other light sources, operating at different wavelengths, have been developed, such as a Tm:Holmium laser functioning at 2 μm and a Cr:ZnSe laser operating at 2.5 μm. Thus, there is an essential need to develop appropriate passive Q-switches that will accommodate the extended eye-safe and other IR (Infrared) laser applications.
U.S. Pat. No. 4,738,798 to Mahler entitled “Semiconductor compositions”, is directed to a composition for particles of a semiconductor material in a copolymer matrix. The copolymer matrix comprises at least one α-olefin having the formula RCH═CH2, where R is selected from hydrogen and straight or branched alkyl groups having from 1 to 8 carbon atoms. The copolymer matrix further comprises at least one α,β-ethylenically unsaturated carboxylic acid having from 3 to 8 carbon atoms and 1 or 2 carboxylic acid groups. The α-olefin content of the copolymer matrix is from about 75 to about 99 weight percent, and the acid monomer content is from about 1 to about 25 weight percent. There is also disclosed a method for preparation of the particles directly in a copolymer matrix, by contacting an ionic copolymer precursor with appropriate anions.
U.S. Pat. No. 5,162,939 to Herron et al entitled “Small-particle semiconductors in rigid matrices”, is directed to small-particle semiconductors immobilized in the pores of a glass matrix. A porous glass contains both a semiconductor and a polymer matrix in its pores. The semiconductor is comprised of any one of a group of materials, such as: CdS, CdSe, ZnS, ZnSe, PbS and PbSe. The polymer is prepared from a monomer which is comprised of any one of a group of materials, such as methacrylate esters and styrene. The glass is comprised of any one of a group of materials, such as: SiO2, GeO2 and TiO2. There is also disclosed a device for producing third-order nonlinear optical effects. The device includes the porous glass containing both a semiconductor and a polymer matrix, at least one cation and at least one anion, and a source of coherent electromagnetic radiation that irradiates the porous glass.
U.S. Pat. No. 6,444,143 to Bawendi et al entitled “Water soluble fluorescent nanocrystals”, is directed to water-soluble nanocrystals that emit light in the visible energy range. A water-soluble semiconductor nanocrystal includes a quantum dot having a selected bandgap energy overcoated with a layer of a material having greater bandgap energy. The quantum dot is a semiconductor nanocrystal with size-dependent optical and electrical properties. The outer layer of the overcoating layer includes a compound. The compound has at least one linking group that attaches the compound to the overcoating layer, and at least one hydrophilic group spaced apart from the linking group by a hydrophobic region, that prevents electron charge across the hydrophobic region. Possible compositions for the compound are provided. The particle size of the nanocrystal core is in the range of 12 Å and 150 Å. The nanocystal preferably has less than a 10% deviation in diameter of the core. The nanocrystal in an aqueous environment preferably exhibits photoluminescence having quantum yields of greater than 10%, where “quantum yield” refers to the ratio of photons emitted to those absorbed.